The present invention relates to a broadcast method for enabling a cell inputted from one input line to be transmitted to a plurality of output lines using a self-routing switch operated in an asynchronous transfer mode (ATM) in an integral broadband service digital network.
Asynchronous transfer mode (ATM) has become important in communication technologies for establishing integral broadband service digital networks which have been studied and developed by the International Telegraph and Telephone Consultative Committee (CCITT) and other organizations.
FIG. 1 shows an example of a system configuration of the broadband switching unit used in the present invention.
A plurality of subscriber terminal equipments 1 such as telephone, TV-telephone, facsimile, and computer terminals are accommodated in a broadband switching unit 2 through a subscriber line 3 and another line 4.
A subscriber terminal equipment TE 1 communicates with a central controller CC 11 in a broadband switching unit 2 through the line 4, and is assigned a virtual path identifier VPI and a virtual channel identifier VCI by the central controller CC 11. The VPI is for determining a path between a terminal terminator for terminating the plurality of subscriber terminal equipments 1 and a switching unit. The VCI is a channel assigned to each of the subscriber terminal equipments TE 1 accommodated in the terminal terminator.
The terminal equipment TE 1 generates a cell containing information to be transmitted to a destination using the assigned VPI and VCI. FIG. 2 shows a conventional cell used in the prior art technology. FIG. 2A shows a cell generated by the terminal equipment TE 1.
The terminal equipment TE 1 divides the information to be transmitted to a destination terminal equipment into pieces of 48-octet user information U. A header H is added to each of the divided pieces of information to generate a 53-octet cell. The header H comprises 4-octet VPI and VCI data, and a one-octet header error control HEC. The terminal equipment TE 1 sends the generated cell to a broadband switching unit through the subscriber line 3.
In the broadband switching unit 2, a subscriber interface SINF 5 receives a cell from the terminal equipment TE 1. After the cells are synchronized, they are transmitted to an ATM interface 6 (ATMIF).
After removing the header error corrector HEC from the 5-octet header in the cell transmitted by the subscriber interface SINF 5, the ATM interface ATMIF 6 adds a 2-octet tag information area to the header, and transmits it to a tag information adder VCC 7 (VCI controller).
The central controller CC 11 generates tag information TAG for indicating a specified destination terminal equipment in the process of communicating with the terminal equipment TE 1 through the line 4, and sends it to the tag information adder VCC 7 through a switch controller SMASH 12 (switch module access shelf).
As shown in FIG. 2B, the tag information TAG is 2-octet information comprising four sets of 4-bit (b.sub.0 -b.sub.3) routing information S (each of the four pieces of routing information is named S.sub.1 -S.sub.4). However, only three bits (b.sub.0 -b.sub.2) of four bits (b.sub.0 -b.sub.3) are significant bits. The remaining bit b.sub.3 is unused.
The tag information adder VCC 7 sets in the tag information area of the cell transmitted from the ATM interface ATMIF 6 the tag information TAG transmitted from the central controller CC 11 through the switch controller SMASH 12. Then, the tag information adder VCC 7 sends the 54-octet cell to a switch SW 9 through a multiplexer/demultiplexer MDX 8.
The switch SW 9 has a configuration in which three self-routing switches SRM (Self Routing Module 20-1, 20-2, and 20-3) are serially connected.
FIG. 3 shows the configuration of self-routing switch SRM 20-N of the prior art which was used as SRMs 20-1 to 20-3. The a self-routing switch SRM 20-N comprises eight input lines 21 (21(0)-21(7)), eight output lines 22 (22(0)-22(7)) and 8.times.8 unit switches USW 23 (23.sub.00 -23.sub.77).
One unit switch USW 23 is provided at eight points on each of eight input lines 21, each point corresponding to one of eight output lines 22. Each switch is assigned a conduction code.
The 54-octet cell transmitted from the tag information adder 7 is applied to the first self-routing switch SRM 20-1. For example, assume that the cell is applied to the input line 21.sub.0 of the first self-routing switch SRM 20-1. The eight unit switches USW 23.sub.00 -23.sub.07 connected to the input line 21.sub.0 extracts the first routing information S.sub.1 from the tag information TAG added to the header H of the inputted cell. Then, each of the switches compares a 3-bit binary number comprising b.sub.0 -b.sub.2 ((000), for example) with the corresponding conduction number C. If the two values match, the corresponding switch USW (23.sub.00, for example) becomes conductive and transmits the cell inputted to the input line 21.sub.0 to the corresponding output line (22.sub.0, for example).
The cell outputted from the output line 22.sub.0 of the first self-routing switch SRM 20-1 is then applied to the input line of the second self-routing switch SRM 20-2. Likewise, in the second and the third self-routing switches SRM (20-2 and 20-3), each of the unit switches USW 23 compares the three bits b.sub.0 -b.sub.2 in the second and the third routing information S.sub.2 and S.sub.3 with its own conduction code C. If these values match, the corresponding unit switch USW 23 .sub.XI (X and I correspond to one of 0-7) transmits the inputted cell to the corresponding output line 22.sub.I.
The cell outputted from the output line 22.sub.I of the third self-routing switch SRM 20-3 is transmitted to the multiplexer/demultiplexer MDX 10. The multiplexer/demultiplexer MDX 10 extracts the fourth piece of routing information S.sub.4 from the tag information TAG added to the header H of the cell transmitted from the switch SW 9, and selects one of the eight output lines.
As described above, the tag information TAG added to an existing cell comprises 4-bit routing information S.sub.1 -S.sub.4, and specifies using a 3-digit binary number one of the eight output lines 22 for each of the self-routing switches SRM 20 and the multiplexer/demultiplexer MDX 10. Therefore, each cell is transmitted to only one output line 22, and cannot realize a broadcast in which a cell is simultaneously transmitted to a plurality of output lines.